Various systems for assisting in the piloting of an aircraft have been proposed, most of which are adapted for fixed-wing aircraft.
U.S. Pat. No. 5,337,048 describes a head-up display (HUD) system in which a diagram is generated comprising superposed bars representing pitch angles (nose up or nose down); U.S. Pat. No. 5,675,328 describes a system displaying two slope scales that cross at a safe attitude value; U.S. Pat. No. 5,614,897 describes a system for displaying departures of a real value from a reference value, concerning the speed and the heading of an aircraft.
French patent FR-2 720 522 describes a collimator connected to an imaging sensor for improving the field of view of a pilot.
U.S. Pat. Nos. 5,057,835 and 5,471,205 describe methods of displaying a chart; French patent FR-2 725 808 describes a collimator displaying a runway axis; U.S. Pat. No. 6,150,960 describes a graphics indicator having symbols representative of actual speed and a variety of characteristic speeds (minimum, maximum, maximum endurance); U.S. Pat. No. 5,343,395 describes a guidance system providing a simulated three-dimensional view of the perimeter of a landing strip.
U.S. Pat. No. 6,107,943 describes a method of displaying quantitative information concerning acceleration (or deceleration) of an aircraft taxiing on the ground.
French patent FR-2 730 841 describes a guidance method comprising displaying an artificial horizon line and displaying a ground speed vector by means of a vertical coordinate for slope angle and a horizontal coordinate for heading angle, these coordinates being calculated from signals or data delivered by an attitude heading reference system (AHRS), an anemo-barometric unit, and a global positioning system (GPS) receiver.
U.S. Pat. No. 6,320,579 describes a system displaying three-dimensional symbols representative of departure from a flight path.
U.S. Pat. Nos. 5,798,713 and 6,054,937 describe methods of representing airplane guidance information comprising a horizon, a previously-calculated trajectory for the airplane, and strips representative of a pitch angle or the presence of another airplane.
U.S. Pat. No. 6,111,526 describes a guidance assistance device that displays a horizon with heading graduations, a line with attitude graduations, a symbol representing the longitudinal axis of the airplane, a speed vector symbol representing the track and the slope followed by the airplane relative to the ground, and a guidance window of position that corresponds to the direction of a point on the desired trajectory that is situated at a distance in front of the airplane.
U.S. Pat. No. 5,420,582 describes a method of representing a three-dimensional view including an estimated line of flight predicted for the airplane, together with the position and the attitude of the airplane; a vertical speed scale is also provided.
U.S. Pat. Nos. 4,419,079 and 5,289,185 describe systems for displaying additional symbols relating to the speed to be followed by the aircraft: in the first patent, the pilot must seek to bring the two potential slope symbols to the level of two incidence symbols by manipulating the throttle, to ensure that the airplane retains a chosen speed; according to the second patent, the total energy to be adopted in order to reach a selected speed is displayed by two identical symbols, and the acceleration or deceleration trend is displayed by two other identical symbols.
U.S. Pat. No. 6,272,404 describes apparatus for displaying a target symbol on a HUD giving the direction of the trajectory to be followed by the helicopter fitted with that apparatus.
A difficulty in showing the horizontal component of the speed of an aircraft by a device for displaying information in superposition on the outside environment, the information comprising in particular a horizon bar, a speed vector, and a target to be reached, lies in the fact that horizontal speed does not correspond directly to a magnitude that can easily be represented geometrically in a three-dimensional space representing the outside world.
Furthermore, in a helicopter, controlling speed is difficult, since acting on the stick for controlling cyclic pitch variation in order to modify speed also leads to coupling with other flight parameters. It is thus not easy simultaneously to follow a horizontal speed and an altitude—or else a horizontal speed and a vertical speed—that are both varying.
In prior art methods and devices for displaying piloting assistance symbols superposed on the outside environment, and that are used mainly in fixed-wing aircraft, speed tracking can be performed in several ways:                the pilot may have an indicator presenting, relative to a speed vector, the potential slope that the aircraft would have if it were traveling at constant speed; if this slope does not coincide with the speed vector, then the aircraft is accelerating or slowing down; the pilot has no information about the speed to be followed, but does have information about speed variation, and it is up to the pilot to know the speed to be adopted on the slope; this indication is well adapted to an airplane in which speed is stabilized all along its approach slope and for which there is no need to manage a large amount of deceleration during descent;        the pilot may have an indication requesting acceleration or deceleration in order to join a reference speed; the pilot then controls this parameter in an open loop; the indicator may be proportional to the power demand needed, and may give the pilot an idea of the amount of control to apply; nevertheless, this indication is not correlated with the dynamics of the aircraft and it needs to be interpreted; this indication is similar to that of a flight director and does not enable the pilot to anticipate actions; this system is much less well adapted to a helicopter than it is to an airplane, since with a helicopter, reducing speed requires action to be taken on two controls (cyclic stick and collective stick) whereas in an airplane, action on the power control alone suffices; and        the pilot may have a conventional speed indicator, showing the present speed and the speed to be reached on a scale or a dial; that kind of indicator is incompatible with the above-mentioned requirements and can mask the outside environment when presented on a HUD.        
Thus, known systems for displaying information for assisting in piloting an aircraft present shortcomings or drawbacks that the present invention seeks to remedy, at least in part.